Fickle winds
What does El Niño mean for climate change, and climate change mean for El Niño?
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On a day like today, it’s hard to comprehend that we live on a warming planet. An icy wind, gusting to severe gale force, seems to be blowing directly from Antarctica. I’m wearing my warmest winter jacket. After a few minutes playing with my dog at the local park, my fingers are freezing. And, according to the calendar, today is the first day of summer.
It’s not the first time in the last few weeks that I’ve dug out my warmest winter clothing. A couple of weeks ago, an even colder southerly blast blew in. It felt like a second winter, in mid-November.
But, while Wellington has certainly experienced some unseasonably chilly weather in the last few weeks, a few cold days don’t mean a trend. There’s nothing unusual about sudden cold weather in spring. When I lived in Christchurch, I can remember it snowing in September and even, once, in October. I can remember some spectacular spring storms in Wellington when half of the people at work were unable to get into the city due to icy roads, flooding or inoperable public transport.
Wellington’s spring was actually neither uncommonly cold, nor uncommonly warm, but in that, it was unusual. Overall, New Zealand has had a warm spring – the fifth warmest on record according to the National Institute of Water and Atmospheric Research. When we have a warm season like this, it’s tempting to see it as connected to climate change, just as, when we get cold weather, it’s easy to doubt. But there’s something else going on with New Zealand’s climate right now – a global climate cycle called the El Niño Southern Oscillation.
The term El Niño will be familiar to most of us from weather reports in the media, but I have to admit that I’ve never quite understood what it meant. Every few years, we hear weather forecasters attributing all sorts of weather phenomena to El Niño, or its opposite La Niña, from storms, to droughts, to unusually warm springs. And it’s exactly that – our warm spring – which has prompted me to learn more about El Niño and La Niña, because four of the five warmest springs that New Zealand has ever experienced have been in La Niña years.
It was Peruvian fishermen who first gave the name “El Niño de Navidad” – the child of Christmas – to what we now understand to be a powerful cycle of wind and sea currents. They observed, perhaps every five or six years, and usually around Christmas, that the anchovy shoals on which they depended would disappear. At the same time, the water would be unusually warm. The combination of absent anchovies and warm water was catastrophic, not just for the fishermen but for all the local marine life. Seabird chicks starved as their parents searched further and further afield for food. Many sea creatures died, from either starvation or the water being too warm, and rotted on the sea surface.
The warm water was the result of changes in ocean currents, but ocean currents don’t change themselves. ENSO starts, not with the sea, but with the wind (the video that I’ve linked to here gives a great graphic explanation). The prevailing winds across the tropical Pacific Ocean blow from east to west – from the coast of South America to Asia. These winds push the warm, tropical water at the ocean’s surface in the same direction, east to west. The effect is strong enough for the sea level in places like Indonesia to be up to half a metre higher than at the South American coast. The movement of the warm water drags cold water up from the ocean’s depths and from the south, in what is known as the Humboldt current. This current of cold water, rich in oxygen and nutrients, makes the waters off the west coast of South America among the most productive on earth. But what it gives to the sea, it takes away from the land, as this same current contributes to the desolate, dry conditions of Chile’s Atacama Desert.
El Niño occurs when the trade winds weaken, and even blow in the opposite direction, which brings the flow of water to a stop and results in warm seas off the South American coast. On the other side of the Pacific, the sea is cooler than usual. These changes in ocean temperatures then affect the atmosphere, not just over the Pacific Ocean, but throughout the tropics and into temperate regions like New Zealand. The atmospheric changes have all sorts of impacts on weather conditions. In New Zealand, El Niño means increased rain in the west, drought in the east and colder winters. In Australia, El Niño leads to warmer temperatures in the south and even more severe droughts in the north and east. In the southern United States, it increases rainfall and flooding, while in the Atlantic Ocean, it reduces the number of hurricanes.
But we aren’t talking about El Niño this year, but its opposite – La Niña. When the prevailing east to west wind intensifies, increasing the easterly flow of warm water, we get a different set of weather conditions entirely.
Under La Niña conditions, there isn’t just more warm water in the western Pacific – some of that water flows down to New Zealand. Not surprisingly, the warmer water brings warmer temperatures, but it also has other impacts. There are fewer north-westerly and more north-easterly winds, which mean more rain for the north of New Zealand, and drier weather in the south.
For the coming summer, La Niña promises warmer temperatures throughout New Zealand, as well as drier conditions south of Nelson and Marlborough, and a wet summer north of Auckland. More worryingly, it is also likely to bring more storms, in the form of ex-cyclones heading down from the tropics.
But while La Niña brings unusually warm weather to New Zealand, the overall pattern of La Niña is actually cooler ocean temperatures. Although the winds push warmer water into certain areas, the windier conditions cool the ocean’s surface and mean more upwelling of cold water from the deep. Warmer ocean temperatures are associated with El Niño rather than La Niña.
Still, the effect of El Niño on global temperatures seems relatively minor compared to climate change. Of the ten warmest years on record, four were El Niño years, three La Niña years and three were normal. There’s no clear pattern linking the El Niño, or La Niña, with record-breaking temperatures. On the other hand, all ten of these warmest years have been in the last two decades. Now that is a clear pattern.
But that does leave the question of what climate change will do to El Niño. On that question, the answer is far from clear. Studies on the link between climate change and El Niño have come up with widely varying conclusions. Some have indicated El Niño will intensify, others that La Niña will intensify, others again have suggested there won’t be much change.
Two studies, using quite different approaches to their research, have come up with a particularly concerning conclusion. A 2014 publication used climate models to predict future El Niño patterns, while a 2019 publication reviewed historical data back to 1901. The 2014 publication predicted that extreme El Niño events were likely to increase under climate change, while the 2019 publication confirmed this prediction by showing that this was already happening.
The studies are concerning, because it is the most extreme of the El Niño events which result in the most extreme impacts. The most recent of these, the 2015/16 El Niño event, saw the most severe coral bleaching on record for the Great Barrier Reef, crop-destroying frosts in Papua New Guinea, drought in places as far afield as the Philippines, Indonesia, Colombia, the Caribbean, Ethiopia and Zimbabwe, and heavy rain causing widespread flooding and landslides in Peru, Ecuador, Tanzania, Kenya and Rwanda.
While the science remains uncertain, one thing which already clear. The worst of these impacts won’t be seen in New Zealand. Just as we see with the persistent conflicts which plague the planet, the worst impacts of El Niño are experienced by the poorest of the world's inhabitants. Droughts will bring hunger. Flooding will bring disease and homelessness. And the impacts of hunger, disease and homelessness will last much longer than the El Niño events themselves.
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